(a) Field of the invention
The present invention relates to a sound projection system for use in diffusing sound of a frequency ranging from as low as 160 Hz up to 5 KHz over a relatively long distance with a substantially constant distribution of the sound over a wide diffusion angle.
(b) Brief description of the prior art
Sound projection systems especially designed for use outdoors or in other environments where sound must be transmitted to substantial distances, are well known in the art.
All of these known systems make use of a channel or duct of gradually increasing cross-section, generally known as "horn", which is fixed to a cabinet enclosing a loudspeaker. In practice, such a channel acts as an acoustic amplifier that increases the usually small vibrating area of the loudspeaker diaphragm at the inlet (or "throat") of the channel into a much larger vibration area at the outlet (or "mouth") of this channel.
It is also well known in the art that if the inlet of the channel is of substantially the same dimension as the diaphragm, losses due to interferences occur with respect to frequencies corresponding to wave lengths at least four times bigger than the size of the inlet.
A standard method for overcoming this problem consists in providing a compression stage between the diaphragm and the inlet of the acoustic channel to increase the air density close to the diaphragm. This standard method is usually carried out by positioning a small perforated wall in front of, and very close to the diaphragm, to form a small compression chamber adjacent this diaphragm. Alternatively and preferably, the above problem is overcome by positioning a stream-lined plug in the middle of the inlet of the channel to substantially reduce its surface area [see for examples, U.S. Pat. Nos. 3,886,710 (CESATI) and 4,181,193 (ISAAC)].
In the former case, the number, the position and dimension of the perforations in the wall must be carefully determined to avoid the generation of resonances of high order in the compression chamber. In the latter case, the external shape of the plug must also be carefully determined to avoid the development of resonances due to the cross-sectional reduction of the channel due to the plug and the direction changes in the passage(s) left available in this channel.
Most of the sound projection systems presently available in the market make use of a stream-lined plug, also known as "face plug", as explained hereinabove, because such a use makes them much easier to manufacture and adjust than those using a perforated wall. The major drawback however with these face-plug equiped, sound projection systems is that they always transmit the sound in the form of a narrow beam that is often annular (see, for example, the preamble of U.S. Pat. No. 4,181,193). None of the existing systems known to the Applicant are actually capable of transmitting or diffusing sound within a wide frequency range with a substantially constant distribution over a wide diffusion angle.